We examined, in three glasshouse experiments, the development of vesicular-arbuscular (VA) mycorrhizas formed by Glomus fasciculatum (Thaxter sensu. Gerd.) Gerd. & Trappe on Trifolium subtẽrraneum L. in relation to plant denser and inoculum quantity. The effect of inoculum placement on the formation of mycorrhizas on Medicago truncatula Gaertn. by another Glomus sp. was studied in a fourth experiment. More mycorrhizal root was formed by Glomus sp. when inoculum was dispersed throughout the soil than when it was localized either near the top or bottom of the pot. The absolute rate of mycorrhiza formation was initially more rapid with localized inoculum. However, a greater weight of mycorrhizal root was subsequently obtained with dispersed inoculum as roots continually intercepted propagules. Infection by G. fasciculatum spread deeper in one soil than in two others, perhaps reflecting differences in root growth and density associated with differences in phosphorus status of the soils for plant growth. The weight of roots which became infected increased with increasing plant density, but not when shoot growth was greatly decreased due to competition between plants. The relative rate of mycorrhizal infection remained unaffected by treatments in each experiment. When a low root density and a high inoculum level were combined, the amount of roots which became infected was greater than expected. These conditions favoured a longer exponential phase in the development of infection.